“We turning left now?” “Yeah, you do know we have a sophisticated biological mechanism for finding that out, right?” “I was just asking for a friend” | Image: Tom Thai

Have you ever seen two fish bump into each other?

The underwater world is an assault of sensory signals. Sound, for example from crashing waves, travels over twice as fast in water than it does in air. Smells, ranging from mating hormones to decaying organisms, clash against each other like instruments in an orchestra and would be overwhelming to us if we had noses sensitive enough to detect them. Light is changing constantly, with the different wavelengths becoming increasingly filtered out with depth until animals are left navigating waters so dark that they would be impossible to penetrate with human eyes.

But despite the chaos, fish are able to identify predators, prey and potential mates with lightning reflexes and take the appropriate action within milliseconds. This is thanks to a super-human sensing mechanism called the lateral line, which gives the fish a sixth sense with which to navigate in their watery habitat.

A subtle stripe of hair can tell fish everything they need to know about their environment | Image: Pogrebnoj Alexandroff

Sometimes visible as a dark strip running along the fish from fin to tail, the lateral line consists of minuscule bundles of hair that can be either attached to the surface of the scales or slightly submerged in channels below the skin. These hairs work similarly to whiskers on a cat, bending in response to any change in the flow of water around the fish. Cells at the base of the hairs then send messages to the brain, containing information on the scale, speed, and direction of the disturbance. This can then be interpreted to give information about the size and shape, and therefore species and likely friendliness, of anything moving in the local area. The ability to identify friend from foe by the flick of its tail is an invaluable tool of survival.

Seas and oceans are one of the least understood habitats on earth, with vast areas being simply too inaccessible to explore. As space on land is becoming rapidly exhausted, we are extending further and deeper into the oceans to source food, generate energy and hunt for new minerals and medicines. Faced with challenges such as the extreme pressure and low temperatures, increasingly we are depending on the work of underwater robots to bring us information on the chemistry, physics, and biology of the deep sea. Engineers looking to improve the performance and capabilities of these underwater robots have found inspiration in the lateral line to improve the performance of these robots.

Researchers in Germany and the US have independently come up with two different systems that recreate the processes that take place in the lateral line hairs. Integrated into underwater robotic technology, these systems could create a robot with much greater perceptions of their surroundings. Traditionally, operators would rely on images from video and sonar to navigate and direct the actions of the robot underwater. Both of these technologies are limited, as they can only provide information on the small area that they are pointing at Imagine only being able to view the world through a toilet roll tube – it’s a bit like that.

Lateral lines could provide a much more detailed 3D picture of the surrounding environment, allowing more informed decisions about how the robot should proceed. The more information available about the surrounding area, including any obstacles, the greater the chance of the robot completing its mission as efficiently and safely as possible. For example, if the battery is running low, the lateral line would be able to identify nearby areas of low water movement where the robot can go to rest and conserve energy out of the current. Taking inspiration from nature, and building robots that can sense like fish, scientists can expect to soon be announcing plenty more exciting discoveries from the mysterious deep ocean.